In order to operate data terminal devices in a packet data network, it is necessary for an address to be assigned to each data terminal device within a network. The address can be attached in each case to data packets transferred over the network in order to enable correct forwarding of the data packets in the network to the intended receiving terminal device. For each connected computer the Internet Protocol Version 4 uses, for example, an address which can be represented as a string of numbers separated by dots, whereby the numbers describe the location of the computer in a hierarchically grouped system in a similar fashion to the digits of the postal code system used by the German Post Office. A hierarchical structure of this type enables a transit node in the network to transport a data packet which is to be forwarded onward in the correct direction without requiring any detailed knowledge of the overall structure of the network in order to do so.
The fact that the addresses in a network of this type reflect the geographical structure of the network results in problems, however, if a terminal device is used at different locations at which it would need to have different addresses as a result of the network structure. Let us assume that a terminal device having the address a1.b1.c1.d1 (where a1.b1.c1.d1 are each natural numbers smaller than 255) is to be operated not by way of its assigned node a1.b1.c1 but at a different location by way of a node a1.b2.c2, then although the terminal device is possibly able to feed data packets into the network, which could also reach their destination, any response from the destination computer to which such a data packet is sent will however be sent by the destination computer to the destination address a1.b1.c1.d1. Since the terminal device is not actually situated at this location, the response will disappear into the void; no communication occurs.
In order to remedy this problem at least in part the mobile IP protocol has been developed. In an adapted form, this protocol is also a component of the Internet Protocol Version 6 standard. This protocol provides that a terminal device is assigned a second address, the so-called care-of address, if it can be logged in the network by way of a node which is not its home node. This care-of address is conveyed to a so-called home agent at the home node of the terminal device. As a result of this the home agent is in a position to intercept data packets intended for the terminal device, in other words those provided with the terminal device's first address, at the home node and to forward them, provided with the care-of address, tunneled to the terminal device at its actual location.
The logging of a terminal device in a packet data network which supports the mobility of terminal devices, such as the mobile IP network mentioned above, involves a considerable signaling requirement which impairs the transport capacity of the network for useful data. On the one hand, when a contact is initiated between the terminal device and the network a free address must be found within the network which can be allocated to the terminal device without any danger of a collision with other terminal devices being operated on the network, while on the other hand this address, once it has been found, must be conveyed to the terminal device in order that the terminal device is able to apply a correct sender's address to packets sent by the terminal device, and the address must be notified to the home agent in order that the home agent is able to correctly forward packets sent to the first address.
This disadvantage is particularly inconvenient if the terminal device is communicating with the packet data network by way of a radio link using a mobile radio network, since in this case transmission bandwidth is scarce—especially for the transmission of information relevant to the functional integrity of the system such as addresses which need to be transmitted with a highly effective level of protection against transmission errors.